1. Field of the Invention
The present invention is directed to a process for the isomerization of cyclopropyl ketones to 2,3-dihydrofurans. More specifically, the process involves the catalytic rearrangement (isomerization) of 1,1-disubstituted-2-hydrocarbylcyclopropanes, wherein one of the substituents in the 1-position is an acyl group, to 2,4,5-substituted-2,3-dihydrofurans.
2. Description of the Prior Art
A procedure for the preparation of substituted dihydrofurans is disclosed in U.S. Pat. Nos. 4,180,446; 4,198,341, 4,198,342 and 4,198,343 to Schmidt. The process involves reacting a .beta.-alkoxycrotonic acid ester or 3,3-bisalkoxybutyric acid ester with a 1,1,1-trihalogen-4-methyl-3-pentene-2-ol or 1,1,1-trihalogen-4-methyl-4-pentene-2-ol in the presence of an acid catalyst. 2,4,4-Trimethyl-3-carbalkoxy-5-(.beta.,.beta.-dihalogenvinyl)-4,5-dihydrof urans are produced in the process.
Alonso et al. in J. Org. Chem., 45, 4530-4532 (1980) report the rearrangement of cyclopropyl ketones to 4,5-dihydrofurans at room temperature using aluminum oxide. The rearrangement is accomplished by passing the cyclopropyl ketone in chloroform through a neutral alumina column. Whereas nearly quantitative yields of the dihydrofuran ##STR1## are obtained, contact times of from 24 to 72 hours are required.
Tishchenko et al. [Chemical Abstracts, 94, (1981), 15229w] describe the ring opening of 1-acetyl-2,2-dichlorocyclopropanes in the presence of sodium alcoholates. Depending on the amount of sodium alcoholate used, a dihydrofuran or alkyne is produced.
Dihydrofurans are also reported by Bahurel et al. in Bull. Soc. Chim. France, 1971 (6), 2203-8 as a by-product from the reaction of 1,4-dichlorobutene-2 and sodium ethyl acetylacetate in an alcoholic solution.
U.S. Pat. No. 4,252,739 to Fayter, Jr. et al. describes a phase-transfer process for the preparation of vinylcyclopropane derivatives by reacting an alkylating agent and an activated methylene compound using an onium catalyst and in the presence of an alkali metal compound and water. Whereas alkylation of the activated carbon atom is predominantly obtained, when the activated methylene compound contains an acyl moiety, e.g. ethyl acetoacetate, up to about 15 percent dihydrofuran by-product can be obtained as a result of alkylation at the oxygen atom of the acyl group.
Dihydrofurans have also been obtained from the rearrangement of acids or light by McGreer et al., Can. J. Chem., 51 (10), 1487-93 (1973); Armitage et al., J. Am. Chem. Soc., 81, 2437-40 (1959); Wilson, J. Am. Chem. Soc., 69, 3002-3 (1947); and Dauben et al., J. Org. Chem. 34, 2301-6 (1969).
While it is known to obtain dihydrofurans from cyclopropyl ketones, it is evident from the above-mentioned references that a variety of other rearrangement products are produced depending upon the reaction conditions employed. Accordingly, it would be highly advantageous if other processes were available whereby dihydrofurans could be specifically obtained by the rearrangement of cyclopropyl ketones.